1. Field of the Invention
This invention relates to apparatus and methods for perforating wells, and more particularly, to a jetting head with a plurality of coplanar jets which are used to penetrate the well casing.
2. Description of the Prior Art
There are a number of methods used in perforating wells which are well known. The present invention overcomes problems associated with these prior methods and provides an apparatus and method which is particularly well suited for, but not limited to, the special situations which are presented in the completion of deviated wells. A brief discussion of several different techniques currently used for the completion of deviated wells follows.
A first, very common manner of completing a deviated well is to case and cement the vertical portion of the well and to leave the deviated portion of the well which runs through the production formation as an open hole, i.e., without any casing in place therein. Hydrocarbon fluids in the formation are produced into the open hole and then through the casing in the vertical portion of the well. The problem with this is there is no case to prevent collapse of the well bore.
A second technique which is commonly used for the completion of deviated wells is to place a length of slotted casing in the deviated portion of the well to prevent the open hole from collapsing. A gravel pack may be placed around the slotted casing. The slotted casing may run for extended lengths through the formation, for example, as long as one mile.
A third technique which is sometimes used to complete deviated wells is to cement casing in both the vertical and deviated portions of the well and then to provide communication between the deviated portion of the casing and the producing formation by means of perforations or casing valves. The formation may also be fractured by creating fractures initiated at the location of the perforations or the casing valves.
In this technique, the formation of perforations is often done using shaped charge methods. That is, explosive charges are carried by a perforating gun, and these explosive charges create holes which penetrate the side wall of the casing and penetrate the cement surrounding the casing. Typically, the holes will be in a pattern extending over a substantial length of the casing.
A problem with the use of explosive charges to perforate is that this method generally creates high damage in the formation by increasing skin and also creating high localized stresses in the formation. By doing this, fractures created by stimulation processes tend to become very tortuous and restrict the production of oil and gas. This problem of tortuosity, literally meaning "marked by repeated twists and bends" reduces the potential production rate of the well because even though the rock moves to open the fracture, severe restrictions still remain.
Tortuosities thus are generally caused by the situation wherein the initial fracture does not coincide with the maximum stress plane. Under such a circumstance, the fracture will twist or bend to finally direct itself to the maximum stress plane. This can be caused by incorrect fracture initiation procedures or high localized stresses which prevent the fracture from initiating properly. An additional problem closely associated with tortuosity is the creation of multiple fractures which will increase leakoff and hence cause screenouts.
When the communication between the casing and production formation is provided by casing valves, those valves may be like those seen in U.S. Pat. No. 4,949,788 to Szarka, et al., U.S. Pat. No. 4,979,561 to Szarka, U.S. Pat. No. 4,991,653 to Schwegman, U.S. Pat. No. 5,029,644 to Szarka et al., and U.S. Pat. No. 4,991,654 to Brandell et al., all assigned to the assignee of the present invention. Such casing valves also provide a large number of radial bore type openings communicating the casing bore with the surrounding formation.
When utilizing either perforated casing or casing valves like those described, fracturing fluid enters the formation through a large multitude of small radial bores at a variety of longitudinal positions along the casing, and there is no accurate control over where the fracture will initiate and in what direction the fracture will initiate. As mentioned, this lack of proper fracture initiation results in tortuosity.
Fracture initiation is largely influenced by the shape and orientation of the initial cavity, maximum and minimum stress direction, near well bore conditions such as localized stresses, or other irregularities that may be encountered such as natural fractures, fossils, etc.
To solve the problems of these prior methods, hydrajetting has been developed. Generally, hydrajetting does not result in skin damage, and no residual stresses occur since jetting is performed at pressures below the yield strength of the rock. Moreover, the jetting tool is positioned in the correct direction for proper fracture initiation. Thus, tortuosities are reduced or eliminated. This is because in hydrajetting, holes are formed by removal of material, rather than compaction. Removal is performed below the compressive strength of the rock, and thus there is no highly stressed area formed. Further, hydrajetting is a slower process. Therefore, temporary deflection or reflection by abnormal positioning will not jeopardize the quality of the cutting process. The main intent of hydrajetting perforating is to be able to position a cavity such that the shape is basically flat and located in the direction of maximum principal stress. By doing this, fractures will start at the edges of such cavities, and tortuosities will therefore not occur.
Examples of hydrajetting perforating tools are disclosed in U.S. Pat. Nos. 5,249,628 and 5,325,923 and U.S. Pat. application Ser. No. 08/206,560, all of which are assigned to the assignee of the present invention. Each of these discloses apparatus and techniques designed to create a cavity which promotes fractures to initiate perpendicular to the well bore, thus being particularly suitable for deviated wells or very shallow vertical wells. These devices are designed for wells drilled in the direction of least principal stress and to create a cavity perpendicular to the well bore.
Jetting parallel to the casing also may be done and involves the movement of the jetting tool up and down the casing. In order to make a cut which is sufficiently deep, the jetting tool must move at a very slow speed. To introduce a good slot in deviated wells, an in-line, multiple jet system must be used.
While such hydrajetting tools substantially reduce the problem of tortuosities in the fractures, tortuosity can still be a problem. This is due to the fact that many operators place their holes randomly, and thus initiate fractures which are uncontrolled. The apparatus and method of the present invention are designed to solve these previous problems by placing the perforations in one plane which is preferably perpendicular to the least principal stress. This is accomplished by placing jets coplanarly and positioning them such that the jets make a cutting angle that is at the steepest possible angle at the contact point in the casing. This improves cutting efficiency through the casing wall.